It is known that magnetic media mass memories currently used may be divided in two broad categories: memory devices such as disk or tape units having a removable media, and fixed media devices.
The fixed media devices are characterized, at a substantial equality in size of the device and the media, by a storage capacity an order of magnitude greater than the removable media devices and by a much greater information access speed.
As a drawback, the magnetic media cannot be removed for replacing it with media containing other information or for its storage in a protected area, such as armored cabinets and like.
This is a severe drawback for all installations where information has to be protected with the utmost security, essentially for secrecy reasons.
The removable media has two advantages for the secrecy purpose: The media may be stored in a protected environment when not used and further the access to information recorded therein is limited to the time the media is installed in the handling equipment.
The risk of abuse in information access is therefore minimized.
To achieve the advantages of intrinsic security offered by devices having removable media it is conceptually possible to implement systems where, rather than removing the recording media, it is the device containing the media which is removed. For instance this is possible in case the device is connected to the system by its own controller unit and an interface dedicated to the controlled device.
However, for cost reasons, the current trend is to use a single controller for a plurality of magnetic recording devices, by means of a common connection interface and connection bus.
The control of the several recording units occurs in "time sharing" and one unit at a time is active in the information exchange with the controller.
This does not mean that the inactive units may be disconnected from the common bus and removed. However inactive, they must guarantee to their input/output terminals, connected to the common bus, a high impedance state in order to avoid the injection of signals on the bus and interference with the signal exchange occurring on the bus.
A partial solution is conceptually possible if the compromise is accepted of performing the removal/insertion of fixed media devices when the system is switched off or when no communication is in progress on the bus among devices and control unit and all devices are inactive.
This too is a serious constraint because it imposes on the bus the blocking of possible transfer process queues waiting to be performed on the bus and completion of the running process.
Only when the bus is inactive, as when all recording devices are inactive, may devices be removed or new devices be inserted.
The apparatus of the present invention overcomes this limitation which enables "hot" removal or insertion of a magnetic recording device on a bus, that is when operations are running on the bus, without interference with the running process.